One aspect of an MMs job is to Make A Market when there is low Vol so what they do is shuffle shares between themselves to give the appearance of liquidity / interest when there's actually little buying pressure.
By walking down the PPS they can possibly pick up shares from shareholders liquidating portions of their holdings or could cause a panic selloff where they will then accumulate dumped shares for a possible PPS run and-or sell them at a profit to interested buyers, if any.
This is not a new concept by any means in that I've literally seen it over and over for the past ~20 years.
Market Data 
Markets 
AI
